A resonant photoelastic modulator (PEM) is an instrument that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term “photoelastic effect” means that an optical element that is mechanically strained (deformed) exhibits birefringence that is proportional to the amount of strain induced into the element. Birefringence means that the refractive index of the element is different for different components of polarized light.
A PEM includes an optical element, such as fused silica, that has attached to it a piezoelectric transducer for vibrating the optical element at a fixed frequency, within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz. The mass of the element is compressed and extended as a result of the vibration.
The compression and extension of the optical element imparts oscillating birefringence characteristics to the optical element. The frequency of this oscillating birefringence is the resonant frequency of the optical element and is dependent on the size of the optical element, and on the velocity of the transducer-generated longitudinal vibration or acoustic wave through the optical element.
Retardation or retardance represents the integrated effect of birefringence acting along the path of electromagnetic radiation (a light beam) traversing the vibrating optical element. If the incident light beam is linearly polarized, two orthogonal components of the polarized light will exit the optical element with a phase difference, called the retardance. For a PEM, the retardation is a sinusoidal function of time. The amplitude of this phase difference is usually characterized as the retardance amplitude or retardation amplitude of the PEM.
Both the size and acoustic wave velocity of a PEM depend on the optical element's temperature. Consequently, the resonant frequency of a PEM will also depend on the device's temperature. In general, this temperature depends on two factors: (1) the ambient temperature, and (2) the amplitude of the stress oscillations in the optical element. At high stress amplitudes, the amount of acoustic (mechanical) energy absorbed in the optical element can become significant. As the absorbed acoustic energy is converted to heat within the mass of the element, significant temperature increases and corresponding shifts in the PEM's resonant frequency can occur.
Thus, even though the retardation amplitude of a PEM can be adjusted at will (within the limits set by the maximum driving voltage provided by the electronic circuits), the system's operating frequency is determined by the PEM's resonant frequency and, as explained above, thus depends on both ambient temperature and the amplitude at which the PEM is driven. This results in an operating frequency that drifts with ambient temperature, as well as during warm-up and after changes in the set retardation amplitude. Such a situation may be undesirable in certain applications where the PEM's operating frequency, as well as its amplitude, must be kept constant.
In view of the foregoing, one can appreciate the value of real-time information indicating the actual performance of the PEM (that is, the particulars of the retardance characteristics induced by the PEM into the light that passes through it). Moreover, this information may be used as feedback control of the PEM to more accurately control the PEM operation.
The present invention is generally directed to a diagnostic system for a PEM. The system provides optically determined information about the retardance characteristics induced by the PEM.
In a preferred embodiment, the diagnostic system is integrated with the PEM so that the PEM performance may be diagnosed or monitored during operation of the PEM. Specifically, the diagnostic system is used alongside an optical setup that employs a primary light beam for conventional purposes such as polarimetry, optical metrology, etc. The diagnostic system includes its own diagnostic light source that is directed through the optical element of the PEM at a location remote from the primary aperture of the PEM. Thus, the diagnostic system and the primary PEM operation can be undertaken simultaneously, with one not interfering with the other.
Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.